The best definition of gastrulation is:

Options

A. Single layered blastula becomes two layered
B. Archenteron is formed
C. Cells move to occupy their definite position
D. Zygote gets converted into larva

Answer

**Step-by-Step Solution:**

1. **Understanding Gastrulation**: Gastrulation is a crucial phase in embryonic development where the single-layered blastula reorganizes into a multi-layered structure. This process is essential for forming the three primary germ layers: ectoderm, mesoderm, and endoderm.

2. **Analyzing the Options**:
- **Option 1**: "Single-layered blastula becomes two-layered."
- This statement is misleading because gastrulation involves the formation of three layers, not just two. The blastula does not simply become two layers; it undergoes a more complex transformation.
- **Option 2**: "Archenteron is formed."
- While the archenteron is indeed a feature of gastrulation, this statement alone does not encompass the full definition of gastrulation. The archenteron is the primitive gut formed during this process, but it does not define gastrulation itself.
- **Option 3**: "Cells move to occupy their definite position."
- This statement accurately describes gastrulation. During this phase, cells migrate and reorganize to form the three germ layers, which is essential for subsequent organ development (organogenesis).
- **Option 4**: "Zygote gets converted into larvae."
- This statement is incorrect as it oversimplifies the developmental process. The zygote undergoes cleavage and forms a blastula before gastrulation occurs.

3. **Conclusion**: After analyzing all the options, the best definition of gastrulation is provided in **Option 3**: "Cells move to occupy their definite position." This reflects the dynamic and organized movement of cells that leads to the formation of the germ layers necessary for further development.

**Final Answer**: The best definition of gastrulation is: "Cells move to occupy their definite position."

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